The present invention relates to mobile communication, and more particularly to methods and apparatuses for efficiently finding a carrier frequency from among a large number of possible carrier frequencies.
Modern mobile communications systems (e.g., cellular telephone systems) rely on wireless media, such as radio technology, to communicate information between communicating parties. Communication is effected by imposing (e.g., via modulation) the desired information (e.g., information representing audio information) onto a carrier frequency. Thus, for communication to take place, the communicating parties must agree on which carrier frequency to use.
The range of useable carrier frequencies is limited, and is therefore allocated according to government regulations. Having allocated a portion of the radio frequency spectrum for use by a particular type of communication technology (e.g., mobile communication devices operating in accordance with any of the IMT-2000 (ITU), Universal Mobile Telecommunications System (UMTS) or Enhanced Data rates for GSM and TDMA/136 Evolution (EDGE) standards, all of which are considered to be third-generation systems), the allocated bandwidth must still be further divided into a number of sub-bands which are allocated for use by different system operators. Depending on the number of operators present in any particular geographic area, each operator may be allocated only a handful of these sub-bands.
Given a channel raster of a given bandwidth and a desired spacing between carrier center frequencies, the number of possible carrier frequencies may be determined. For example, the UMTS system calls for a defined 2xc3x9760 MHZ bandwidth used for Frequency Division Duplex (FDD) to be divided up into 5 MHZ-wide carriers, thus there are 60/5=12 of them. A raster is defined of 200 kHz, which allows 25 subcarrier positions within a 5 MHZ bandwidth. Thus, there are 12xc3x9725=300 possible carrier positions.
Although 300 possible sub-carriers may be defined, it is anticipated that only a small sub-set of them are actually required. For example, in a UMTS, FDD system, only one fifth of the potential sub-carriers (each 200 kHz wide) are expected to be used independent of operator or location, due to frequency regulations defined in each country. However, a mobile terminal that has lost contact with a system (e.g., by having been powered off, or by having moved out of range of a previously serving system) needs to regain contact, and this is done by locating a sub-carrier frequency that is being used by a nearby system. One strategy for doing this has the mobile terminal scanning all of the possible 300 sub-carriers until it finds one that is in use. However, this can be a lengthy process. Considering that only one fifth of the possible sub-carriers would actually be in use, it is apparent that the process of setting out to scan all of the sub-carriers wastes not only time, but also energy, which is a precious commodity in a battery-powered device.
It has been proposed to address this problem by supplying mobile terminals with information that identifies, for each known service area, the sub-carriers that are in use. One way of supplying this information is by broadcasting it to the mobile terminal, which then stores it in a memory. In operation, whenever a mobile terminal loses contact with a service provider, it first xe2x80x9cguessesxe2x80x9d its present location, and uses this guess to retrieve a list of sub-carriers that are known to be in use in the guessed location. The sub-carriers in the list are then scanned by the mobile terminal. If the guess was correct, then a sub-carrier should be found with a reduced amount of effort. However, if the guess is incorrect, then the initial scanning operation may not turn up a sub-carrier, in which case the mobile terminal may have to resort to scanning all possible sub-carriers until one is found. Thus, this technique suffers from the problem of only being as good as the mobile terminal""s guess and that it is practically impossible to have complete information about all the service areas. However, many mobile terminals are routinely turned off just prior to travel, for example on aircraft. Thus, a mobile terminal can hardly be expected to be very accurate in guessing where it is when it is turned on.
Another solution is to predefine possible carrier positions, and thus reduce the number of carriers that have to be scanned. This would give less flexibility, and would probably degrade the overall capacity.
Yet another proposed solution involves allocating a common channel that, in every service area, provides information identifying which sub-carrier frequencies should be searched on first priority. Such a solution provides a perfect scanning, and therefore requires only a minimum of search time. However, this solution requires that operators world-wide agree on a common solution for a world-wide logical or physical pilot channel, and that the operators be willing to provide in their common channel parts, information about competitive systems.
So far, the above examples have focused on examples that refer to the FDD part of the UMTS (also known as the xe2x80x9cpaired bandxe2x80x9d). However, the same issues arise as well in connection with the Time Division Duplex (TDD) part of the UMTS band (also known as the xe2x80x9cunpaired bandxe2x80x9d).
Because of the difficulty in obtaining universal agreement on these issues, it is desirable to find yet another approach that will reduce the scan time performed by mobile terminals and that can be based on the Universal Terrestrial Radio Access (UTRA) concept itself.
It should be emphasized that the terms xe2x80x9ccomprisesxe2x80x9d and xe2x80x9ccomprisingxe2x80x9d, when used in this specification, are taken to specify the presence of stated features, integers, steps or components; but the use of these terms does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved in methods and apparatuses for operating a mobile terminal. This includes storing identifiers of sub-carrier frequencies known to be in use without regard to operator identity or location of use. A first scanning of only sub-carrier frequencies identified by the stored identifiers is performed to find a used sub-carrier frequency. If no used sub-carrier frequencies are found in the first scanning, then a second scanning of sub-carrier frequencies that are not associated with stored identifiers is performed.